bennyk
The matter of the 'fifty years' in Josephus may simply be one of the following:
scribal error
Quoting Berossus in error
Dating an event regarding circumstances in the Land whilst desolated marking 50 years within the overall time period of 70 years. You can pick any one of these scenarios.
The prophecies of Jeremiah most certainly refer to a 70 years period of desolation a 70 year period of Exile and a 70 year period of Babylonian domination/servitude all three events to run concurrently.
The beginning of the 70 years from 609 BCE as you claim is impossible historically and makes no sense because nothing of any relevance happened in that year.
Josephus states that the Temple lay in a state of obscurity for fifty years not desolation for he earlier said that the city was desolate during the interval of seventy years.
scholar